1. Field of the Invention
The present invention relates to equipment and a method for controlling the operation of a line of machines, particularly presses, which are intended to perform a succession of operations on pieces which are advanced along the line, the line also including a plurality of programmable manipulator robots interposed between the machines for transferring the pieces from one machine to another, each machine having a device which is movable between an operative position for carrying out a certain operation on a piece situated in the machine and an inoperative position in which a piece to be worked can be deposited in the machine or a piece worked by the machine can be picked up; each manipulator robot including a gripping member which is movable between a first position in which it can grip a piece situated in the machine immediately upstream of the robot and a second position in which it can deposit the piece it has gripped in the machine immediately downstream of the robot.
Lines of the type indicated above are used in particular in the automotive industry for the manufacture of pressed sheet-metal motor-vehicle-bodywork components. Each sheet-metal element is subjected to a series of drawing, blanking and perforating operations, etc. until the finished piece is produced.
2. Description of the Prior Art
For some time Applicant has produced and sold a programmable manipulator robot which is particularly advantageous for transferring pieces from one press to another in a line of presses of the type indicated above. This manipulator robot is described and illustrated in Italian Utility Model IT-U-0 195 537, in the corresponding German Patent Application DE-A-3 445 003, in the corresponding French Patent Application FR-A-2 556 274 and in the corresponding U.S. Pat. No. 4,661,040.
The use of robots in lines of the aforesaid type obviously provides the advantage that the line has considerable flexibility which enables it to be adapted quickly and easily to new production requirements.
Before the use of robots, the conventional technique used was that of so-called transfer lines in which all the machines in the line worked in synchronism and the various pieces situated in the line were advanced simultaneously by one step along the line during the interval between two successive stages in which the machines in the line were active. The introduction of robots as the means for transferring the pieces from one press to another, as indicated above, has provided flexible lines but, at the same time, has lead to the asynchronous operation of the various machines in the line. In other words, in press lines with robots between the presses, each machine forming part of the line (presses, robots, piece-loaders upstream of the line and piece-unloaders downstream of the line) has an operating cycle which is not perfectly linked to the operating cycles of the other machines. In order for the line to operate normally, the robots must, therefore, also fulfil a regulatory function by pausing in their travels between one press and another when necessary. As a result, the operating cycles of the robots are generally uneven, with frequent stops and consequent decelerations and accelerations. The rate of operation of the line is not well defined since it is subject to variations caused by the variations in the cycles of the individual machines over a period of time or by phenomena of instability and lack of synchronisation.